1. Field of the Invention
The present invention relates to a Low Noise Block down-converter (hereinafter referred to as LNB), and more particularly, to an LNB in a satellite broadcast receiving system.
2. Description of the Background Art
In the satellite broadcast receiving system, an LNB has a function of amplifying, with low noise, a satellite broadcast wave signal in a 12 GHz band received from a broadcast satellite, and also frequency-converting the signal to be in an intermediate frequency (IF) band. The frequency conversion described above is performed in a mixer (hereinafter referred to as MIX) within the LNB.
FIG. 9 is a circuit diagram of an MIX. The MIX circuit includes a high electron mobility transistor (hereinafter referred to as HEMT) 50 used as a transistor for actually performing frequency conversion, and a driving circuit 10. Driving circuit 10 includes a PNP bipolar transistor Tr1 and a plurality of resistance elements.
Driving circuit 10 supplies a constant voltage to the drain of HEMT 50, and the constant voltage allows the MIX to perform stable frequency conversion. That is, characteristics of the MIX largely depend on the drain voltage of HEMT 50.
However, in the MIX, when an ambient temperature of PNP bipolar transistor Tr1 within driving circuit 10 is changed, the temperature characteristic of PNP bipolar transistor Tr1 itself causes a change in the voltage supplied to the drain of HEMT 50. The change in the drain voltage causes a change in the gain frequency characteristic of HEMT 50, interfering stable frequency conversion.